(a) Technical Field
The present disclosure relates to a method of preparing a nano-sized transition metal catalyst in a carbon support, which can be used as an electrode material or the like of a fuel cell.
(b) Background Art
Fuel cells directly convert chemical energy produced by oxidation of fuel into electrical energy, thereby improving fuel economy, reducing emission and providing environmentally friendly properties in automobile-related field.
A proton exchange membrane fuel cell (PEMFC) and a direct methanol fuel cell (DMFC), which are typical fuel cells, include a catalyst material resulting from dispersing transition metal nanoparticles containing platinum as an electrode material on a carbon support. However, because platinum is expensive and rare, researches have been conducted to reduce the amount of platinum or replace platinum with a less expensive or cheap metal.
In the case where platinum is applied only on the surface layer of nanoparticles, the electronic energy level of surface platinum atoms is lowered. Thereby, the problem of the fuel cell with increased oxygen reduction overvoltage may be solved, as was reported in recent research (Stamenkovic, V. R. etc., Science, vol. 315, p. 493). Thus, in the application of platinum only on the surface layer of nanoparticles, there are required various methods of synthesizing metal nanoparticles supported on carbon powder having a narrow particle size distribution and a high degree of dispersion as a core material.
Examples of the transition metal usable as the electrode material of the fuel cell include 4d transition metals (Ru, Rh, Pd) and 5d transition metals (Os, Ir, Pt). A method of preparing nanoparticles of binary or ternary alloys, as well as the above single metal, is traditionally based on a borohydride reduction method. Although the borohydride reduction method using water as a solvent is advantageous because of the simplicity of the process, it is problematic in that the nanoparticles present on the surface of the carbon support may agglomerate severely, and in some cases, the nanoparticles may not be formed on the surface of the carbon support. In the case where platinum is reduced along with another metal, the synthesis results are reported to be comparatively good, which is considered to be due to the high reduction potential of platinum.
Another method to prepare nanoparticles is a polyol method. This method, which is typically used to prepare platinum or ruthenium nanoparticles, includes applying heat to an alcohol solvent such as ethylene glycol or propylene-1,2-glycol thereby causing a dehydrogenation reaction so that the dissolved metal precursor is reduced. However, this method is disadvantageous because the added metal precursor is difficult to be completely reduced or nanoparticles having a large amount of oxide in lieu of pure metal are frequently prepared due to the use of sodium hydroxide (NaOH) as an additive, which may deteriorate electrochemical activity.
In addition, a method of using ethanol as a solvent and sodium borohydride (NaBH4) as a reducing agent to preparing transition metal nanoparticles supported on the surface of carbon powder is known (U.S. Pat. No. 7,335,245, Korean Patent No. 10-823094, Japanese Unexamined Patent Publication No. 2000-000467), but this method is not suitable for industrial mass production as it requires high amount of energy for the reduction process.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.